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XLIX. Letter from Ezekiel Walker, Esq. containing 

 further Remarks on his new Transit Instrumsnt. 



To Mr. Tilloch. 



I SIR, 

 N a paper, which was printca- in the 25th volume of the 

 Philosophical Magazine, p. 173, I descnbed a method of 

 constructing an instrumcni for observing the sun's transit 

 over the meridian upon a much smaller scale than that which 

 I had formerly used, yet sufficiently exact for regulating 

 common clocks and watches ; but it now appears, from an 

 instrument that has lately been erected according to those 

 directions, that the method deserves further consideration. 

 A gentleman has communicated to me five observations, 

 made with this instrument, on the sun's passage over the 

 meridian ; and the greatest difference between any two of 

 them amounts to no more than one second and six tenths. 

 These were the only observations that this gentleman had 

 taken at the time he communicated them to mc, and not 

 chosen out of a great number: they were taken with a 

 chronometer, which was immediately compared with ob- 

 servations made on the same day with a transit telescope. 

 Hence I am inclined to believe, that with some improve- 

 ments in the construction of this instrument, and in the 

 mode of using it, the sun's transit may be determined still 

 nearer the truth. 



When the utmost precision is required, it will be neces- 

 sary to protect the instrument from the extremes ot heat 

 and cold, by laying the foundation of the pier about two or 

 three feet below the surface of the ground, and from that 

 depth erect a small building over it, at the distance of eight 

 or ten inches, leaving an empty space all round. This 

 building should be carried up higher than the gnomon a; 

 the south, Ijut it need not be more than six or eight inches 

 higher than the stone upon which the meridian line is drawn 

 at the soulli. 



If the plane on which llie line is drawn should dip either 

 to the norlh or to the south, the correctness of the instru- 

 inent will not be affected by it ; nor v/\\\ a small inclination 

 east or west alter the position of the line iu.df, but it will 

 •- Vol. 27. No. lOB. May 1807. T aircct 



